This invention relates to a limited slip differential adaptable as an intra-axle differential intermediate the two wheel-driving components of a single drive axle assembly on a self-propelled vehicle for appropriately distributing torque to the driving components in instantaneous response to varying torque demands therebetween, and/or as an inter-axle differential intermediate the front and rear drive axle assemblies on a vehicle having two drive axles for appropriately distributing torque to the two axle assemblies in response to the varying torque demands therebetween.
The necessity of a differential between two mechanically driven coaxial wheels of a shelf-propelled vehicle has long been established, and the desirability of an ideal limited slip differential in this and other applications has long been recognized.
The automotive driving axle differential in most common use at this time is essentially the same as the original one invented some one hundred and fifty years ago. Its characteristics allow the vehicle to negotiate turns and uneven terrain and to compensate for differences in the rolling radii of the driving wheel tires while maintaining equal torque distribution to the two driving wheels. The algebraic sum of the rotational velocity of the two axle shafts is always equal to the rotational velocity of the differential housing. These characteristics are usually recognized as the reasons why the design has endured the evolution of the self-propelled vehicle.
The disadvantages of the conventional differential are also equally well recognized. Tractive conditions inadequate to support locomotion encountered by one wheel of the pair limits the usable torque distributed to the opposite wheel to the same magnitude, and the wheel with the lesser traction spins, stalling the progress of the vehicle. Also, during periods of adequate traction, if one driving wheel should bounce and lose contact with the road surface, that wheel accelerates while in the air and slips upon subsequent reengagement with the road surface causing tire scuffing, heat build-up, and excessive wear due to cutting from contact with sharp-edged objects it would normally be expected to roll over without damage.
Many experts in the fields of power train technology and vehicle performance have analyzed these problems, and the following statements appear to form a consensus of what is required.
The ideal vehicle drive differential should:
1. Allow normal differential action to occur as when compensating for uneven terrain or unequal tire rolling radii, or when cornering.
2. Function automatically and promptly in response to transient variations in tractive conditions encountered by either of the vehicle's driving wheels.
3. Allow maximum utilization of available traction.
4. Cause no adverse effect on the stability or on the handling characteristics of the vehicle.
5. Prevent wheel spinning when either driving wheel traction diminishes as long as the opposite wheel has adequate traction.
Obviously, there are other aspects to be considered. The design should be simple, practical, and economical to produce and assemble. It should be strong and reliable with adequate service life expectancy, and it should not require periodic servicing or adjustments. It should operate efficiently and, therefore, not be wasteful of energy and it should be quiet.
While some of the prior art differentials have satisfied some of the above criteria, none are known to have complied completely, and most have caused additional problems in one or more of the categories enumerated. An example of this would be a limited slip differential comprising one or more spring-loaded friction-type clutch disc packs which impede the normal rotation of the side differential gears, restricting differential action to a pre-set torque level. Below this pre-set torque level, no differential action can occur, with consequent tire scuffing and interference with normal steering capability of the vehicle. Above this pre-set torque level, differential action occurs, but it requires more power to cause it to occur, and the slipping clutch discs generate heat and wear rapidly, causing frequent replacement and/or gradual reduction in the pre-set torque level, accompanied often times with noise known as "chatter."